How does a holding’s weight change its contribution to a portfolio loss?

Set a clean calculation boundary

Use starting market values, one common measurement period, no borrowing, no contributions, no withdrawals, and no trades during the period. Measure all holdings in the same currency. Those conditions keep the arithmetic transparent; they do not describe every real portfolio.

Investor.gov’s allocation guide emphasizes that the mix of holdings matters to portfolio risk. The exercise below isolates one part of that mix: the amount allocated to a holding. It does not calculate a probability, a volatility contribution, or a recommended position limit.

Calculate a small position’s loss

Invent a $10,000 portfolio with $1,000 in Holding A and $9,000 in other holdings. A therefore starts at 10% of the portfolio. Stipulate that A falls 50% while everything else is unchanged. A loses $500 and the ending portfolio is $9,500.

The portfolio return is −$500 ÷ $10,000 = −5%. Equivalently, 10% × −50% = −5 percentage points. Saying the portfolio lost 50% would confuse the individual position’s return with its contribution to the whole.

Compare a larger position

In a separate hypothetical portfolio, put $3,000 of the same $10,000 total into Holding B. B starts at 30%. If B falls 20% and all other holdings are unchanged, the dollar loss is $600 and the portfolio ends at $9,400, down 6%.

B’s assumed decline is smaller than A’s, yet its contribution is larger because its starting weight is larger. The comparison makes no statement about which asset is more likely to decline or which position is suitable. Both loss scenarios were chosen for arithmetic, not estimated from data.

Combine contributions without double-counting

Now construct one $10,000 portfolio containing $1,000 in A, $3,000 in B, and $6,000 in unchanged holdings. Under the same simultaneous losses, A loses $500 and B loses $600. The portfolio ends at $8,900, down 11%. The two contributions, −5 and −6 percentage points, sum to −11.

This addition uses the original weights and a common period. If you instead use weights measured after the losses, or introduce trades midway through the period, the simple calculation no longer describes the same scenario. Keep amounts and dates beside the percentages.

Keep the units clear: a five-percentage-point contribution is measured against the starting portfolio, while a 50% decline is measured against A’s own starting value. The denominators answer different questions.

Method, sources, and limits

The original examples were checked with decimal arithmetic. Investor.gov’s source was opened on 26 October 2026 for allocation context. No real holding, expected loss, correlation, liquidity assumption, or maximum tolerable loss was assessed. The arithmetic omits taxes, fees, leverage, and currency changes. A scenario loss contribution is an accounting result under chosen conditions, not a forecast or a statistical risk contribution.